This invention relates to an improved process for preparing asymmetric isoindoline pigments having high brilliance and color strength and good light- and weather-fastness.
Compounds of formula I are valuable organic pigments previously described in the literature. Such compounds are disclosed, for example, in U.S. Pat. Nos. 3,991,054, 3,923,806, 3,987,045, 4,166,179, 4,262,120, 4,316,023, 4,400,507, 4,401,815, 4,599,113, 4,764,217, and 4,970,310 and in Canadian Patent 1,140,130. When describing preparative methods, however, the references disclose processes require an initial preparation of 1-amino-3-iminoisoindoline (also known as 1,3-diiminoisoindoline) (formula A) ##STR2## or ring-substituted derivatives thereof from 1,2-dicyanobenzene and ammonia. Cf. F. Baumann et al., Angew. Chem., 68, 133-150 (1956). Thus, the technical and economical disadvantages of the disclosed methods include the need to convert 1,2-dicyano-benzene into this intermediate in an additional step when compared to the process of the present invention, coupled with safety and environmental problems associated with the need to handle ammonia. In addition, the processes disclosed in these references are typified by the isolation of all intermediates.
The use of dialkoxy isoindolines instead of 1-amino-3-imino-isoindoline avoids the need for ammonia. For example, U.S. Pat. Nos. 4,426,533 and 4,500,718 disclose the use of dialkoxy intermediates analogous to formula VIII (see below) prepared by the reaction of 1,2-dicyanobenzenes with alcoholate bases (formed in situ from a strong base in an alcohol). These patents, however, do not disclose the preparation of purely asymmetric compounds. Instead, the references describe a method for preparing symmetric bis(cyanomethylene) compounds by reaction of dialkoxy intermediates with a single cyanomethylene compound or a mixture containing both asymmetric and symmetric bis(cyanomethylene) compounds by reaction with a mixture of two different cyanomethylene compounds.
U.S. Pat. Nos. 3,646,033, and 3,794,659 disclose a stepwise method for using 1-amino-3-imino-isoindoline, dialkoxy isoindolines, and other such reactive isoindoline derivatives to prepare asymmetric isoindoline derivatives. These patents, however, teach the importance of removing impurities by isolating intermediates and thus, do not suggest methods in which intermediates are not isolated. E.g., Example 76 and Example 25 (note reference to German Patentschrift 879,102, which teaches isolation of 1-alkoxy-3-iminoisoindoline).
Therefore, an object of the present invention was to develop a procedure for synthesizing isoindoline pigments of formula I in pure asymmetric form and without the use of ammonia and 1-amino-3-imino-isoindoline and without the need to isolate intermediates.